feat(sim): board feature parity (reset/GPS/sensors/admin/keyboard) + input fixes

Rounds out the browser sim harness with the rest of the physical board's
interactions: a reset button (JS-driven, since board.reboot() is inert
under -sEXIT_RUNTIME=0), GPS input wired into a real LocationProvider via
new SimSensorManager, JS-settable battery/environment telemetry, an
admin/repeater-login test hook (sendRoomLogin against the default
"password"), and full physical-keyboard text entry (printable ASCII
passthrough into the existing KeyboardWidget, Tab->KEY_KB_ENTER submit).

Also fixes three real bugs found while exercising all of this in a real
browser:
- UITask.cpp's native-only stdin poll branch had no __EMSCRIPTEN__
  exclusion, so it also compiled into the wasm build and called a real,
  blocking window.prompt() on nearly every frame -- the actual cause of
  the reported time/controls jumping. Now gated to native only.
- 'n'/'p' were mapped as Next/Prev keyboard shortcuts, colliding with
  typing those literal letters. Removed the shortcuts; added explicit
  Next/Prev buttons to mesh.html (previously relied solely on them).
- Buttons grabbed native browser keyboard focus on click, so a later
  stray Enter/Space could silently re-trigger a previously-clicked button
  (e.g. Reset). mousedown now calls preventDefault() on all buttons.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

variants/sim/target.h

@@ -11,6 +11,13 @@
 #include "SimMainBoard.h"
 #include "SimRTCClock.h"
 #include <helpers/SensorManager.h>
+// SimSensorManager.h before SimLocationProvider.h, deliberately: the
+// `sensors` global's real type is SimSensorManager (defined in
+// target.cpp), and SimLocationProvider.h's own `extern SimSensorManager
+// sensors;` declaration (see that file's comment) needs the complete type
+// already visible -- C++ requires every declaration of the same global to
+// agree on its exact type, not just something covariant/compatible.
+#include "SimSensorManager.h"
 // Included here (rather than only where it's used) so its JS-facing
 // sim_location_set() EMSCRIPTEN_KEEPALIVE export actually gets compiled
 // into every SIM_PLATFORM target that includes target.h (companion_radio
@@ -25,7 +32,7 @@
 extern SimMainBoard board;
 extern SimRadio radio_driver;
 extern SimRTCClock rtc_clock;
-extern SensorManager sensors;
+extern SimSensorManager sensors;
 
 #ifdef DISPLAY_CLASS
   extern DISPLAY_CLASS display;